1. Field of the Invention
The present invention relates to control of a clock signal, and more particularly, to a clock signal selecting apparatus and method that guarantee the continuity of an output clock signal.
2. Description of the Related Art
Frequency synchronization is necessary between data transmission/communication systems and between devices in each of the data transmission/communication systems. Especially, for a high-speed/large-capacity data communication system or a high-precision frequency hierarchy-based data transmission system, frequency synchronization with the help of a timing device, such as a network synchronous device, is more than a requirement.
In addition, devices in a cheap small capacity data communication system may operate asynchronously or synchronously after their frequencies are synchronized with one another. For example, a switch fabric, which is an element of a data communication system, needs to synchronize a clock signal for a switch card with a clock signal for a line card so as to enhance the reliability of a serial link between the switch card and the line card.
In order to synchronize clock signals output from different systems or different devices in each of the different systems, a dual timing apparatus is used to provide a synchronization clock signal in consideration of the stability and reliability of each of the different systems. However, the dual timing apparatus includes two timing devices disposed in different places and inevitably has a complicated circuit structure in order to synchronize clock signals output from the two timing devices with each other. In this regard, the dual timing apparatus is not cost-effective, and it is difficult to distribute the clock signals from the two timing devices along the same path.